RFID reader

ABSTRACT

An RFID reader comprises a receiver section for receiving an RF signal and a transmitter section for transmitting an RF signal. The reader is a multicarrier RFID reader and both the transmitted RF signal and the received RF signal comprise at least two frequencies.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of United Kingdom Patent Officeapplication No. 0709313.1 GB filed May 15, 2007 and of United KingdomPatent Office application No. 0802055.4 GB filed Feb. 05, 2008, whichare incorporated by reference herein in their entirety.

FIELD OF INVENTION

The mass application of passive ultra high frequency (UHF) radiofrequency identification (RFID) systems is becoming widespread in theretail industry, logistics and commerce. In most applications, it isvery important that all tags are interrogated successfully during aninventory round, and no tags are missed. Despite efforts in improvingthe performance of the current RFID systems, 100% success rate has notyet been reached in some applications.

BACKGROUND OF INVENTION

In passive RFID systems the tags receive their power from the RF fieldtransmitted by the reader. The efficiency of the power transfer from thereader to the tags is essential for reliable operation.

It is well know in the art, that radio wave propagation suffers frommultipath effects. In the presence of reflections, the power received bythe tags varies over location and frequency. If the received power atthe tag falls below the sensitivity threshold of the tag, the tag willbe missed from the inventory. Typical sensitivity of the tags today is−15 dBm to −20 dBm.

There are two commonly used methods to minimize the effect of multipathpropagation in RFID systems, space diversity and frequency hopping.

SUMMARY OF INVENTION

A typical gate reader employs several antennas that provide spacediversity. First, the tags are interrogated from a first antenna. Themajority of the tags, which receive adequate power, will respond.However, the tags which are in the nulls of the interference pattern aremissed from the first reading cycle. Next, the tags are interrogatedfrom a second antenna, which is located in a different position. Fromthe second antenna, the multipath pattern is different and those tagswhich were in a null previously are now likely to be energizedadequately. The interrogation is repeated, switching all antennas in asequential fashion and therefore maximising the read probability.

In addition to space diversity, in some systems, frequency hopping isalso used to mitigate the multipath effects even further. In this casethe tag population is interrogated at a certain frequency F1 first.After a pre-set time, when the interrogation is completed at F1, thereader is switched to a different frequency F2. At F2, the multipathpattern is different from that of F1 and those tags which were missedduring the first reading cycle are now likely to be read. Theinterrogation is repeated, switching the reader to different frequenciesin a sequential fashion and therefore maximising the success rate.

In the frequency hopping system, the tags are illuminated one frequencyat a time. The consecutive reading cycles at different frequencies areperformed sequentially. This means that if there are N frequencies, andthe time taken for a reading cycle is R, the total inventory time isT_(inv)=N*R.

Reading speed and success rate is a major competitive parameter for RFIDsystems. This is particularly the case in fast moving applications, orin cases where the tags are buried in the shadow of conductive items andtherefore the illumination time is relatively short.

In accordance with a first aspect of the present invention an RFIDreader comprises a receiver section for receiving an RF signal and atransmitter section for transmitting an RF signal; wherein the reader isa multicarrier RFID reader and both the transmitted RF signal and thereceived RF signal comprise at least two frequencies.

In the present invention, unlike in the frequency hopping systems, thetags are now illuminated at several frequencies simultaneously. Thisshortens the inventory time substantially, because the reading isperformed at the several frequencies parallel. The total inventory timefor a multicarrier reader is therefore T_(inv)=R.

Preferably, the receiver section comprises an input to receive an RFsignal; a down converter stage to downconvert the RF signal to baseband;and a processor.

Preferably, the downconverter stage further comprises a splitter tosplit the received RF signal into its component frequency signals; andwherein each component signal is downconverted.

Alternatively, in a digital implementation, the RF signal undergoes afirst downconversion stage using an RF carrier local oscillator, then asecond downconversion stage for each individual carrier frequency.

Preferably, a down converter is provided for each of the split RFsignals.

Preferably, the reader further comprises a baseband filter to filter outbeat frequencies of the received, downconverted signal.

Preferably, the transmitter section comprises a baseband signalgenerator, an upconverter stage and an RF output.

Preferably, the upconverter stage further comprises a splitter to splita signal from the baseband signal generator into at least twofrequencies; and wherein upconversion of each baseband signal isperformed.

Alternatively, in a digital implementation, a first upconversion stageis carried out using a local oscillator at each individual carrierfrequency, then a further upconversion stage is carried out using an RFcarrier frequency local oscillator.

Preferably, an up converter is provided for each of the split basebandsignals.

Preferably, the upconverter stage further comprises a summer to combinethe upconverted signals for transmission.

Preferably, the baseband signal is generated in a baseband digitalsignal processor.

Preferably, the reader is a hand portable receiver.

Preferably, a single antenna is used for transmission and reception ofthe RF signals.

Alternatively, the reader is a gate reader.

Preferably, the gate reader comprises multiple antennas.

Preferably, the reader incorporates frequency hopping functionality.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of an RFID reader according to the present invention will nowbe described with reference to the accompanying drawings in which:

FIG. 1 illustrates an example of an RF implementation of an RFID readeraccording to the present invention;

FIG. 2 shows an example of a digital implementation of a multicarrierreader according to the present invention; and,

FIG. 3 is a graph of amplitude against distance from tag to reflectingsurface, showing how power transfer in a typical multipath environmentis improved using the RFID reader of FIG. 1.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 shows an example of a multicarrier reader according to thepresent invention in which the reader transmits and receives on twofrequencies simultaneously. This example shows an implementation of themulticarrier architecture in the analog (RF) domain. A transmit basebandsignal is generated in a digital signal processor (DSP) section 1 andthen applied 2 to the digital to analogue converter 3. Up to this pointthe functionality of the multicarrier reader is the same as that in thesingle carrier architecture.

The baseband signal is split 4 and up-converted to F1 and F2 frequenciesusing two quadrature up-converters 5, 6 which are fed with F1 and F2signals from respective local oscillators 7, 8. The up-converted signalsare amplified 9, 10 summed 11 and and output 12 applied to an antenna13.

For simplification, in FIG. 1 the complex I and Q paths are not shownseparately. Each device is duplicated in the actual transceiverprocessing two independent, I and Q channels.

The transmitted signal containing F1 and F2 frequency components arrivesat tags (not shown). The tags have no means of distinguishing betweenthe different frequencies and therefore ‘see’ an input signal which hasthe power of both waves propagating at F1 and F2 frequencies.

There is a beat note present at the input of the tag at a frequency ofF2−F1. This frequency needs to be filtered out after rectificationwithin the tag. Ordinary RFID tags already have such low pass filteringfor both energy storage and for interference rejection. However, the lowpass filtering time constant may need to be increased to smooth out thebeat note of the multi-carrier signal. The tags backscatter the receivedCW signal at both F1 and F2 frequencies.

In the receiver, the incoming RF signal 14 that contains F1 and F2components is split with a power divider 15. A quadrature down-converter16 down converts the input signal to baseband I and Q components, usingthe local oscillator 7 that operates at F1 frequency. A seconddownconversion is also performed with a second quadrature down-converter17 and the local oscillator 8 that operates at F2 frequency. Thebaseband signals are summed up with a combiner 18.

A low pass filter 19 is provided which has an additional role comparedto anti-aliasing filters used in single carrier readers. In themulticarrier receiver a beat note is present in the baseband at afrequency of F2−F1. This needs to be removed before the signal isapplied to the analogue to digital converter. At the output 20 of thelow pass filter 19, the baseband signal contains only the I Q componentsof the back-scattered signal emitted by the tags at both frequencies.From here, the architecture of the reader is similar to that in othersingle carrier applications. An analogue to digital converter (ADC) 21converts the output 20 to a digital signal 22 which is processed in adigital signal processor 23.

The invention is not restricted to transmitting two carriers only.Generally, as the number of carriers and the occupied bandwidthincrease, the protection against multipath effects improves. Increasingthe number of carriers and the bandwidth of the reader minimises thedepth of the nulls. For large number of carriers it may be advantageousto generate the multi-carrier signal in a baseband DSP. For this,complex mixers and numerically controlled oscillators may be used toshift the signals to different frequencies and then sum up the F1,F2 . .. Fn components. In this case only a single up and down-conversion isnecessary in the RF domain, but the DSP and the analogue to digitalconversion becomes more difficult. An example of an implementation of amulticarrier architecture in the digital domain is shown in FIG. 2.

In this example, the RF input 14 is down converted in down converter 40using an RF carrier frequency local oscillator 41. The downconvertedsignal 42 is passed through a low pass filter 43 and afterdown-conversion, the entire RF bandwidth of the signal 44 is digitizedwith a broadband analogue to digital converter (ADC) 45. The individualcarriers are then further down-converted 47 to DC using numericallycontrolled oscillators (NCO's) 48 tuned to their respective carrierfrequencies (F1, F2 . . . Fn. The baseband signals 49 are combined 50 inthe DSP receiver 23 in a coherent fashion.

In the DSP transmitter 1, several carriers 51 are modulated 52 with datausing their respective NCOs 48. These signals 53 are summed 54 and thenconverted to the analogue domain in a digital to analogue converter 55.The analogue signal passes through a low pass filter 56 and themulticarrier signal is then up-converted 57 to the final RF frequencyusing the local oscillator 41. The upconverted signal is furtheramplified in power amplifier 58 and output 59. The power amplifieramplifies an amplitude variant signal and therefore needs to be linear.

The benefits of the present invention can be seen in FIG. 3. PassiveRFID devices operate over a short range of up to 5 m. In such anenvironment, the delay spread is very short. This means that substantialimprovements in power transfer can be achieved even with a very fewcarriers. In FIG. 3, the relative path loss is simulated between thereader and the tag in a typical multipath environment. A reflectingsurface is placed 2 m away from the tag and the tag is moved along theinterrogation field from a position of 2 m to 3 m away from thereflecting source. In a single carrier case, the graph 30 shows thedepth of the nulls is −35 dB. With two carriers this is improved to −18dB for 10 MHz, shown in the graph 31; −12.5 dB for 20 MHz, shown in thegraph 32 and −9 dB for 30 MHz spacing shown in the graph 33.

Assuming that the tags operate down to −20 dB relative amplitude, in thesingle carrier case in three locations the tags would be missed. Bycontrast, with two carriers spaced even 10 MHz apart the tags would bealways read. This is a considerable improvement in power transfer andsuccess rate, giving a reader with better reliability and performancethan conventional readers.

A multicarrier reader with one antenna may be provided for hand-heldapplications. A multicarrier reader with multiple antennas may beprovided for gate readers. In either case, a multicarrier reader mayinclude frequency hopping as well.

Frequency hopping does not effect the architecture, but does alters theway in which the NCO's are controlled. Without frequency hopping, allthe NCO's 48 are tuned permanently to their individual carrierfrequencies. In the case of frequency hopping, the carrier frequenciesare also changing from time to time. By altering the way the carriersare combined from slot to slot, the nulls of the multipath responses arereduced further. In future, further RF frequencies may be allocated forRFID applications. Should the analogue bandwidth becoming available forfrequency hopping exceed the bandwidth of the ADC and DAC devices, forthese cases the frequency hopping would be implemented in the analoguedomain. In this case the local oscillator 41 is tuned from slot to slotand the NCOs 48 remain fixed.

1-15. (canceled)
 16. A RFID reader, comprising: a receiver section forreceiving an RF signal; and a transmitter section for transmitting an RFsignal, wherein the reader is a multicarrier RFID reader, wherein atransmitted RF signal and a received RF signal comprise at least twofrequencies.
 17. The reader according to claim 16, wherein the receiversection comprises an input to receive an R-F signal, a down converterstage to downconvert the RF signal to a baseband, and a processor. 18.The reader according to claim 17, wherein the downconverter stagefurther comprises a splitter to split the received RF signal into itscomponent frequency signals; and wherein each component signal isdownconverted.
 19. The reader according to claim 18, wherein a downconverter is provided for each of the split RF signals.
 20. The readeraccording to claim 17, further comprising a baseband filter to filterout beat frequencies of the received, downconverted signal.
 21. Thereader according to claim 16, wherein the transmitter section comprisesa baseband signal generator, an upconverter stage and an RF output. 22.The reader according to claim 21, wherein the upconverter stage furthercomprises a splitter to split a signal from the baseband signalgenerator into at least two frequencies; and wherein upconversion ofeach baseband signal is performed.
 23. The reader according to claim 22,wherein an up converter is provided for each of the split basebandsignals.
 24. The reader according to claim 22, wherein the upconverterstage further comprises a summer to combine the upconverted signals fortransmission.
 25. The reader according to claim 22, wherein the basebandsignal is generated in a baseband digital signal processor.
 26. Thereader according to claim 16, wherein the reader is a hand portablereceiver.
 27. The reader according to claim 16, wherein a single antennais used for transmission and reception of the RF signals.
 28. The readeraccording to claims 16, wherein the reader is a gate reader.
 29. Thereader according to claim 28, wherein the gate reader comprises multipleantennas.
 30. The reader according to claim 16, wherein the readerincorporates frequency hopping functionality.